A Belgian man is going to the European Court of Human Rights after his depressed mother was killed by lethal injection under the country's liberal euthanasia laws.

In a case that raises new questions over the extent of so-called "mercy killings", Tom Mortier's mother Godelieva De Troyer (64) was killed by lethal injection at her own request in a Brussels hospital in April 2012.

Doctors agreed to terminate her life after ruling that she suffered from "untreatable depression", despite the fact that she was physically healthy.

The killing horrified her son, who was not contacted until after his mother had died, when a hospital rang asking him to retrieve her body from the morgue. Euthanasia has become increasingly common in Belgium, where the medical establishment has swung behind the view that it can be used to relieve psychological as well as physical suffering.

Often, the person exercising the right to die opts for a "party atmosphere" in their final moments, celebrating with a final glass of champagne or a favourite piece of music with loved ones.